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22 CHAPTF,FDESCENTWITH MODIFICATION: A DARWINIAN VIEW OF LIFE OUTLII\'E Historical{ontext for Evolutionary Theory A. Western culture resistedevolutionary views of life B. Theories of geological gradualismhelped clear the path for evolutionary biologists l. II. C. Lamarck placed fossils in an evolutionary context The Darwinian Revolution A. Field researchhelped Darwin frame his view of life'. scienceas a process B. The Origin of Speciesdevelopedtwo main points: the occurrence of evolution and natural selectionas its mechanism III. Evidence of Evolution A. Evidenceof evolution pervadesbiology B, What is theoretical about the Darwinian view of life? OBJECTTVES After reading this chapter and attending lecture,the student should be able to: State the two major points Darwin made in The Origin of Species concerning the 1. E,arth'sbiota. Z. Compareand contrastPlato'sphilosophyof idealismand Aristotle'sscala naturae. DescribeCarolusLinnaeus'contributionto Darwin'stheory of evolution. 3. 4. DescribeGeorgesCuvier'scontributionto paleontology' Explain how Cuvier and his followers usedthe concept of catastrophismto oppose 5. evolution. Explain how the principle of gradualism and Charles Lyell's theory of uniformitarianism influencedDarwin's ideas about evolution. Describe Jean Baptiste Lamarck's model for how adaptationsevolve. 7. Describehow CharlesDarwin usedhis observationsfrom the voyage of the HMS Beagle 8. to formulate and support his theory of evolution' g. Describehow Alfred Russelwallace influencedcharles Darwin. "descent with 10. Explain what Darwin meant by the principle of common descentand modification". 1 1. Explain what evidenceconvincedDarwin that specieschangeover time. 12. State, in their own words, three inferences Darwin made from his observations, which led him to proposenatural selectionas mechanismfor evolutionarychange. 6. 13. 14. 15. 16. Explain why variation was so importantto Darwin's theory. Explain how ReverendThomasMalthus'essayinfluencedCharlesDarwin. Distinguishbetweenartificial selectionand naturalselection. Explain why the populationis the smallestunit that can evolve. 326 Unit IV Mechanisms of Evolution 11. Using some contemporary examples, explain how natural selection results in evolutionary change. Explain why the emergenceof populationgeneticswas an important turning point for evolutionary theory. Describethe lines of evidenceCharlesDarwin usedto supportthe principle of common descent. 18. 19. 20. 21. 22. Describehow molecular biology can be usedto study the evolutionary relationships among organisms. Explain the problem with the statementthat Darwinism is 'Just a theory". Distinguishbetweenthe scientificand colloquialuse of the word "theory". KEYTBRMS evolution fossils natural selection descentwith modification artificial selection sedimentaryrocks evolutionary adaptations paleontology natural theology gradualism biogeography homology taxonomy homologous+tructures uniformitarianism vestigialorgans ontogeny phylogeny LECTT]RENOTBS Evolution, the unifoing theme woven throughout the text and course, refers to the processesthat have transformed life on earth from its earliest forms to the enorrnous diversity that characterizes it today. The first convincing case for evolution was published in a book by Charles Darwin on November 24,1859.In this book, On the Origin of Speciesby Means of Natural Selection,Darwin: ' Synthesized seemingly unrelated facts into a conceptual framework that accounts for both the unity and diversity of life. . Discussedimportant biological issuesabout organisms, such as why there are so many kinds of organisms,their origins and relationships,similaritiesand differences,geographic distribution, and adaptationsto their environment. . Made two major points: 1. Speciesevolved from ancestralspeciesand were not specially created. 2. Natural selection is a mechanismthat could result in this evolutionary change. I. Historical Context for Evolutionary Theory A. Western culture resisted evolutionary views of life The impact of Darwin's ideaspartially dependedupon historicaland social context(see Campbell,Figure 22.1). . Darwin's view of life contrastedsharply with the acceptedviewpoint: the Earth was only a few thousand years old and was populated by unchanging life forms made by the Creator during a single week. . Thus, On the Origin of Species by Means of Natural Selection not only challengedprevailing scientific views, but also challengedthe roots of Western culture. Chapter22 DescentWith Modification:A DarwinianYiew of Life 327 t . The scale of nature and natural theology Many Greek philosophersbelieved in the gradual evolution_of life. However, the two ihat influenced Western culture most, Plato (427 347 B-C.) and his student inconsistentwith a concept were which Aristotle (384 3228.C.), held opinions of evolution. plato, whose philosophy is known as idealism (essentialism),believedthat . there were two coeiisting worlds: an ideal, eternal, real world and an illusionary imperfect *orid that humans perceive with their senses' T o Plato, + Variations in plant and animal populations were merely imperfect representativesof ideal forms; only the perfect ideal forms were real. Evolution would be counterproductive in a world where ideal organisms were already perfectly adaptedto their environments' Aristotle questionedthe Platonic philosophy of dual worlds, but his beliefs also excludedevolution. =+ Recognizing that organismsvary from simple to complex, he believed that they could be placed on a scale of increasing complexity (scala naturae); on this ladder of life, each form had its allotted rung and each mng was occuPied' = In this view of life, specieswere fixed and did not evolve' = The scala naturae view of life prevailed for over 2000 years. + . The creationist-essentialistdogmathat specieswere individually createdand fixed became embeddedin Western thought as the Old Testamentaccount of creation from the Judeo-Christianculture fortified prejudice against evolution. . Natural Theology,a philosophy that the Creator'splan could be revealed by studying nature-, dominated European and American biology even as Darwinism emerged. . For natural theologians, adaptations of organisms were evidence that the Creator had designedevery speciesfor a particular purpose' . Natural theology's major objective was to classiff speciesrevealing God's createdstepson the ladder of life. carolus Linnaeus (1707 - 1778),a Swedishphysician and botanist, sought order in the diversity of life ad maiorem Dei gloriam (for the greater glory of God). naming and classifying of . Known as the father of taxonomy-the nomenclature still used binomial of system organisms-he developedthe todaY. He adopted a system for grouping speciesinto categories and ranking the categoriesinto a hierarchy. For example, similar speciesare grouped into a g"nus; similar generaare groupedinto the sameorder' Linnaeus found order in the diversity of life with his hierarchy of taxonomic categories. . The clustering of speciesin taxonomic groups did not imply evolutionary relationships to Linnaeus, since he believed that specieswere permanent creations. . Linnaeus, a natural theologian, developedhis classification scheme only to reveal God'splan and even stated Deus creavit, Linnaeus disposuit ("God creates,Linnaeus arranges")' . 328 Unit IV Mechanisms of Evolution 2. Cuvier, fossils, and catastrophism Fossils : Relics or impressionsof organismsfrom the past preservedin rock . Most fossils are found in sedimentaryrocks, which: Form when new layers of sand and mud settle to the bottom of seas, lakes, and marshes, covering and compressing older layers into rock (e.g. sandstoneand shale) = May be deposited in many layers (strata) in places where shorelines repeatedlyadvanceand retreat. Later erosion can wear away the upper (younger) strata,revealing older stratawhich had been buried. ' The fossil record thus provides evidence that Earth has had a successionof flora and fauna(seeCampbell,Figure 22.2). The study of fossils,paleontology, was founded by the French anatomistGeorges Cuvier (1769-1832)who: . Realized life's history was recorded in fossil-containing strata and documentedthe succession of fossil speciesin the ParisBasin ' Noted each stratum was characterizedby a unique set of fossil species and that the older (deeper) the stratum, the more dissimilar the flora and fauna from modern life forms . Understoodthat extinction had been a common occurrencein the history of life since, from stratum to stratum, new species appeared and others disappeared Even with paleontological evidence,Cuvier was an effective opponent to the evolutionistsof his day. . He reconciled the fossil evidencewith his belief in the fixity of speciesby speculating that boundariesbetween fossil strata correspondedin time to catastrophicevents, such as floods or droughts. . This view of Earth's history is known as catastrophism. Catastrophism : Theory that major changesin the Earth's crust are the result of catastrophicevents rather than from gradual processesbf change Cuvier explained the appearanceof new speciesin younger rock that were absent from older rock by proposing that: . Periodic localizedcatastrophes resultedin massextinctions. . After the local flora and fauna had become extinct, the region would be repopulatedby foreign speciesimmigrating from other areas. = B. Theories of geological gradualism helped clear the path for evolutionary biologists In the late l8th century, a new theory of geological gradualism gained popularity among geologiststhat would greatly influence Darwin. Gradualism: Principle that profound changeis the cumulative product of slow, continuousprocesses . Competed with Cuvier's theory of catastrophism ' Proposedby JamesHutton (1975), a Scottish geologist. He proposedthat it was possible to explain the various land forms by looking at mechanismscurrently operating in the world. Example: Canyons form by erosion from rivers, and fossil-bearingsedimentary rocks form from particles eroded from the land and carried by rivers to the sea. with Modification:A DarwinianView of Life 329 chapter22 Descent gradualism into Charles Lyell, a leading geologistof Darwin's time, expandedHutton's the theory known as uniformitarianism' and have operated Uniformitarianism : Theory that geological processesare uniform from the origin of the Earth to the present that their . It was Lyell's extreme idea that geologicalprocessesare so uniform rates and effects must balanceout through time' the erosion . Example:processesthat build mountainsare eventuallybalancedby of mountains' that Darwin rejecteduniformitarianism,but was greatly influencedby conclusions Lyell: and Hutton of followed directly from the observations slow, gradual . The Earth must be ancient. If geological change results from older than much processesrather than sudden"u.it., then the Earth must be inference' the 6000 years indicatedby many theologianson the basisof biblical . Very slow and subtle processespersisting over a great length of time can cause substantialchange. C. Lamarck placed fossils in an evolutionary context along with Earth's Several18th centurynaturalistssuggestedthat life had evolved (1809) a and LamaiJt 0744-1829)_deve.l.oped published changes.Only Jean'eaptiste comp"rehensivemodelwhichattemptedtoexplainhowlifeevolved. History Museum in Lamarck was in charge of the invertebratecollection at the Natural Paris,which allowed him to: identiff several . Compare modern speciesto fossil forms, and in the process, to younger fossils older of series lines of descentcomposedof a chronological fossilsto modern sPecies' (as opposed to . Envision many laddersof life which organismscould climb movement)' Aristotle's single ladderwithout which were = The bottom rungs were occupied by microscopic organisms continuallygeneratedspontaneouslyfromnonlivingmaterial. + At the tops of the ladderswere the most complex plants and animals' toward increasing Lamarck believed that evolution was driven by an innate tendency complexity, which he equatedwith perfection' adapted to . As organisms attained perfection, they became better and better their environments. sentiments . Thus, Lamarck believed that evolution respondedto organisms' interieurs ("felt needs"). which included Lamarck proposeda mechanismby which specific adaptationsevolve, fwo related princiPles: cope with the l. (Jse and disuse. Those body organs used extensively to deteriorate' environment become larger an-dstrongerwhile those not used organism acquired 2. Inheritance of acquired characteristics.The modifications an during its tifetimJcould be passedalong to its offspring' credit for Although his mechanismof evolution was in error, Lamarck deserves proposing that: the extant . Evolution is the best explanation for both the fossil record and diversitY of life. . The Earth is ancient' 330 Unit IV Mechanisms of Evolution . tr. Adaptation to the environmentis a primary product of evolution. The Darwinian Revolution At the beginning of the l9th century, natural theology still dominated the European and American intellectualclimate.In 1809,the sameyear Lamarck publishedhis theory of evolution, CharlesDarwin was born in Shrewsbury,England. ' Though interestedin nature,Charles(at l6) was sent by his physician father to the University of Edinburghto study medicine, which he found boring and distasteful. ' He left Edinburgh without a degree and enrolled at Christ College, Cambridge University to prepare for the clergy. + Nearly all naturalists and other scientists were clergymen, and a majority held to the philosophyof natural theology. =+ Charles studied under the Reverend John Henslow, a botany professor at Cambridge,and receivedhis B.A. degreein 1831. = Professor Henslow recommendedhim to Captain Robert FitzRoy who was preparing the survey ship HMS Beagle for an around the world voyage. A. Field research helped Darwin frame his view of life: science ils a process l. The voyage of the Beagle The HMS Beagle, with Darwin aboard,sailed from England in December 1831 (see CampbellFigure 22.3). ' The voyage's mission was to chart the poorly known South American coastline. ' While the ship'screw surveyedthe coast, Darwin spent most of his time ashorecollecting specimensof the exotic and diverse flora and fauna. While the ship worked its way around the continent, Darwin observedthe various adaptationsof plants and animalsthat inhabitedthe diverseenvironmentsof South America: Brazilianjungles, grasslands of the Argentinepampas,desolateislandsof Tierra del Fuego, and the Andes Mountains. Darwin noted the following: ' The South American flora and fauna from different regions were distinct from the flora and fauna of Europe. ' Temperate specieswere taxonomically closer to speciesliving in tropical regionsof South America than to temperatespeciesof Europe. ' The South American fossils he found (while differing from modern species) were distinctly SouthAmerican in their resemblanceto the living plants and animals of that continent. Geographicaldistribution was particularly confusing in the caseof the fauna of the Galapagos,recentlyformed volcanic islandswhich lie on the equatorabout 900 km west of South America. Most animal specieson the Galapagosare unique to those islands, but resemblespeciesliving on the SouthAmericanmainland. Darwin collected l3 types of finches from the Galapagos,and although they were similar, they seemedto be different species. =+ Somewere uniqueto individual islands =+ Others were found on two or more islandsthat were close tosether Chapter22 DescentWith Modification:A DarwinianView of Life 331 ,, By the time the Beagle left the Galapagos,Darwin had read Lyell's Principles of Geology,and was influencedby Lyell's ideas. . Darwin had begunto doubt the church's position that the Earth was static and had been cieated only a few thousandyears before' . when Darwin acknowledgedthat the Earth was ancient and constantly changing, he had taken an important step toward recognizing that life on Earth had also evolved. Darwin focuses on adaPtation Darwin was not sure whether the l3 types of finches he collected on the Galapagos were different speciesor varietiesof the samespecies' . After he returnedto Englandin 1836, an ornithologist indicatedthat they were actuallYdifferent sPecies. observationsmade during the voyage and in 1837 began the He reassessed first notebookon the origin of species' Darwin perceivedthe origin of 1ew^speciesand adaptationas closely related processes;new speciescould arise from an ancestralpopulation by gradually accrmulaiing adaptationsto a different environment. For example, . Two populationsof a speciescould be isolated in different environments and divergeas eachadaptedto local conditions' . Over many generations,the two populations could become dissimilar enoughto be designatedseparatespecies' . This is apparentlywhat happenedto the Galapagosfinches; their different (See beaksare adaptationsto speilfic foods availableon their home islands. . Campbell,Figure22.4) By the early 1840s,Darwin had formed his theory of natural selection as the mechanism-ofadaptiveevolution,but delayedpublishingit' . Reclusiveand in poor health, Darwin was well known as a naturalist from the specimers un^dletters he had sent to Britain from the voyage on the Beagle. . He frequently correspondedand met with Lyell, Henslow, and other scientists. In 1844,Darwin wrote a long essayon the origin of speciesand natural selection' . He realizedthe importance and subversivenature of his work, but did not in nublish the information becausehe wished to gather more evidence iupport of his theorY. . Evolutionary thinking was emerging at this time, and Lyell ad-monished Darwin to pubtisn on ih. subjectbefore someoneelse published it first. as a In June 1858, Darwin receiveda letter from Alfred Wailace, who was working specimencollector in the East Indies. . Accompanying the letter was a manuscriptdetailing Wallace'sown theory of natuial ieleition which was almost identical to Darwin's. . The letter asked Darwin to evaluatethe theory and forward the manuscript to Lyell if it was thoughtworthy of publication' "smashed." . Darwin did so, althoughhe felt that his own originality would be . Lyell and a colleaguepresentedWallace'spaper along with excerpts from Darwin's unpublisliedi844 .rruy to the LinnaeanSocietyof London on July 1, I 858. 332 Unit IV Mechanisms of Evolution Darw n #*,:',l: z:;,:f:; ;!,::ff : T*fJl':T :iJ"H r: lT: deve,opedan d supportednatural selection much more extensivelythan wallace. Darwin's book and its proponents quickly convinced the majority of biologiststhat biodiversityis a product of evolution. Darwin succeededwhere previous evolutionists had failed not only because sciencewas moving away from natural theology, but becausehe ionvinced his readerswith logic and evidence. B. The Origin of Speciesdeveloped two main points: the occurrence of evolution and natural selection as its mechanism l. Descent with modification Darwin used the phrase"descentwith modification,"not evolution.in the first edition';:::::,::# ^(Y;';'r;ure with an organisms rerated throughdescent rrom some unknown ancestralpopulationthat lived in the remote past. Diverse modifications (adaptations) accumulatedover millions of years, as descendantsfrom this common ancestormoved into various habitais. Darwin's metaphor for the history of life was a branching tree with multiple branchingfrom a common trunk to the tips of living twigs, symbolic of the o'""j"'"oi1:ilt?T.?"Tl#il1T;, isanancestrar popuration common to ail evolutionary lines of descentbranching from that fork. Speciesthat are very similar sharea common ancestorat a recent branch point on the phylogenetictree. Less closely relatedorganismssharea more ancientcommon ancestorat an earlier branch point. Most branchesof evolution are dead ends since abofi 99yo of all species that ever lived are extinct. To Darwin, Linnaeus' taxonomicschemereflectedthe branchinggenealogyof the tree of life. ' It recognized that the diversity of organismscould be ordered into "groups -levels subordinateto groups", with organisms at the different taxonomic relatedthrough descentfrom common ancestors. ' Classificationalone does not confirm the principle of common descent,but when combinedwith other lines of evidence,the relationshipsare clear. ' For example, genetic analysis of species that are thought to be closely related on the basis of anatomical features and other criteria reveals a common hereditary background. 2. Natural selection and adaptation Darwin's book focusedon the role of naturalselectionin adaptation(seeCampbell, Figure 22.5). Ernst Mayr of HarvardUniversitydissectedthe logic of Darwin'i theory into three inferencesbasedon five observations: ' Observation.l: All specieshave such great potential fertility that their populationsizewould increaseexponentially if all individualstLat are born reproducedsuccessfully. ' Observation 2: Populations tend to remain stable in size, except for seasonalfluctuations. With Modification:A DarwinianView of Life 333 Chaoter22 Descent . . . Environmentalresourcesare limited' Observation3: can . Inference.l: Production of more individualsthan the environment a of individuals among existence sJpport leads to a struggle for population,withonlyafractionofoffspringsurvivingeachgeneration. in their Observation4: Individuals of a population vary extensively alike' exactly are no two individuals characteristics; Observation5'. Much of this variation is heritable' . Inference 2: Survival in the struggle for existence is not random, but dJpends in part on the herediiary constitution of the surviving individuals.Tiose individualswhose inherited characteristicsfit them less best to their environment are likely to leave more offspring than fit individuals. . Inference J: This unequalability of individualsto survive and reproduce will lead to a gradual change in a population, with favorable accumulatingover the generations' characteristics SummarizingDarwin's ideas: . Natural selectionis this differential successin reproduction, and its product is adaptationof organismsto their environment' . Natural selectionoccurs from the interaction between the environment and the inherent variability in a population' . Variations in a population arise by chance,-butnatural selection is not a for chance ph.noln"non, since environmental factors set definite criteria reproductivesuccess. Darwin was already aware of the strugglefor existencecausedby overproduction' Thomas when he read an essayon human population written by the Reverend Malthus (1798). . In this esEay,Malthus held that much of human suffering was a consequence of human populationsgrowing faster than the food supply' only a . This capacity for overproduction is common to all species, and of offspring leave and fraction of new individuals complete development their own; the rest die or are unable to reproduce' possible. Variation and overproduction in populationsmake natural selection genes on to more . On the average, the most fit individuals pass their offspring than less fit individuals' over . This results from environmentalediting, which favors some variations others. that natural selection From his experienceswith artificial selection,Darwin inferred could causeiubstantial changein populations' . Through the breeding of domesticatedplants and animals, humans have with desired modified speciesover many generationsby selectingindividuals traits as breeding stock. to their . The plants and animals we grow for food show little resemblance 22'6)' Figure (see Campbell, wild ancestors . if such changecould be achievedby artificial selection that reasoned Darwin in a relatively short period of tim-e,then natural selection should be capable of of consideruLt" *odincations of species over hundreds of thousands generations. 334 UnitrV Mechanisms of Evolution ' Even if the advantagesof some heritable traits over others are slight, they will accumulate in the population after many generations of natural selection eliminating less favorable variations. Gradualism is fundamentalto the Darwinian view of evolution. Darwin reasoned that: ' Life did not evolve suddenlyby quantum leaps, but instead by a gradual accumulationof small changes. ' Natural selection operating in differing contexts over vast spans of time could account for the diversity of life. SummarizingDarwin's view of evolution: ' The diverse forms of life have arisen by descent with modification from ancestralspecies. ' The mechanism of modification has been natural selection workinc gradually over long periods of time. a. Some subtleties of natural selection Populations are important in evolutionary theory, since a population is the smallestunit that can evolve. Population = A group of interbreedingindividuals belonging to a particular speciesand sharing a common geographicarea Natural selection is a consequenceof interactionsbetweenindividual organisms and their environment, but individuals do not evolve. ' Evolution can only be measuredas change in relative proportions of variations in a population over several generations. ' Natural selection can only amplifr or diminish heritable variations. ' Organismscan adapt to changes in their immediate environment and can be otherwise modified by life experiences, but these acquired characteristicscannot be inherited. ' Evolutionists must distinguishbetween adaptationsan organism acquires during its lifetime and those inherited adaptations that evolve in a population over many generationsas a result of natural selection. Specifics of natural selectionare situational. ' Environmental factors vary from area to area and from time to time. ' An adaptationunder one set of conditions may be uselessor detrimental in different circumstances. b. Examples of natural selection in action In an effort to test Darwin's hypothesisthat the beaks of Galapagosfinches are evolutionary adaptationsto different food sources,Peter and Rosemary Grant of Princeton University have been conducting a long-term study on medium ground finches (Geospizaforlls) on Daphne Major, a tiny Galapagosisland. They have discovered that: ' Average beak depth (an inherited trait) oscillates with rainfall (see Campbell,F igure 22.7). + In wet years, birds preferentially feed on small seeds,and averagebeak depth decreases. 9 In dry years, small seeds are less plentiful, so survival dependson the finches being able to crack the less preferred larger seeds.Average beak depth increasesduring dry years. View of Life 335 A Darwinian With Modification: 22 Descent Chapter , lt can be inferred that the change in beak depth is an adaptive response to the relative availability of small seedsfrom year to year' 'n'1"u#,::;':::";,::;":,:;::i:#l"ffrYf :T"lll"'"";,,."nmen context may not work in another. , Beak evolution on Daphne Maior does not result from inheritance of acquired characterisiics. The environment did not create beaks specializedfor large or small seeds, but only acted on inherited variations already present in the population. The proportion of finchesincreasedduring dry periodsbecause,on average, thicker-beaked thicker-beakedbirds transmitted their genesto more offspring than did birds. thinner-beaked Michael Singerand Camille Parmesanof the University of Texas,have evolutionaryadaptationin a butterfly population (Edith's documented"rapid checkerspot)livinginameadownearCarsonCity,Nevada. . In only a decade, this butterfly population ap_parently adapted to changingvegetationby inheritedchangesin reproductivebehavior. . Femaleslay eggs preferentially on certain plants which.provide food for the larvae after they hatch. fn 1983, checkerspotslaid about 80% of their eggs on a native plant, Collinsia parviflora' . By 1993, the butterflies were laying -about 70o/o.of their eggs on Piantago lanceolata, an invading weed from surrounding cattle ranches' . The researchersdemonstratedthat the switch in plant preference is genetic; daughters of butterflies that deposited !9$s on Plantago inherited the taste for that plant, choosing it over Collinsla when they laid their eggs. There are hundredsof examplesof natural selectionin laboratorypopulations of such organismsas Drosophila. Other examplesof natural selection in action include: . Antibiotic resistancein bacteria(seeCampbell,Chapter 18) . Body size of guppies exposed to different predators (see campbell, Chapter 1) m. Evidence of Evolution A. Evidence of evolution pervades biology an Darwin used severallines of evidenceto supporthis principle of common descent, lend biology, molecular from those evolutionarychange.Recentdiscoveries,inciuding supportto his evolutionaryview of life' 1. BiogeograPhY It was biogeographicalevidencethat first suggestedcommon descentto Darwin, b"cuure tn? Ul"oglographicalpatternshe observedonly made sensein the light of e v o l u t i o n' Biogeography:Thegeographicaldistributionofspecies Islandshave many endemicspecieswhich are closely relatedto specieson the nearestmainlandor neighboringisland.Somelogical questionsfollow: 336 unirIV Mechanisms of Evolution . ' ' J Why are two islandswith similar environments in different parts of the world not populated by closely related species,but rather by species more closely related to those from the nearest mainland even when that environment is quite different? Why are South American tropical animals more closely related to South American desert animals than to African tropical animals? Why does Australia have such a diversity of marsupial animals and very few placental animals even though the environment can easily support placentals? The fossil record Darwin was troubledby the absenceof transitionalfossilslinking modern life to ancestralforms. . Even though the fossil record is still incomplete, paleontologistscontinue to find importantnew fossils,and many key links are no longer missing. . For example, fossilized whales link these aquatic mammals to their (seeCampbell,Figure 22.8). terrestrialpredecessors Although still incomplete,the fossil record provides informationthat supports other types of evidence about the major branchesof the phylogenetic tree. For example: . Prokaryotes are placed as the ancestorsof all life by evidence from cell biology, biochemistry,and molecularbiology. . Fossilevidenceshowsthe chronological appearanceof the vertebratesas being sequentialwith fishesfirst, followed by amphibians,reptiles and then birds and mammals.This sequenceis also supportedby many other types of evidence. 3 . Comparative anatomy Anatomical similaritiesamong speciesgroupedin the sametaxonomiccategoryare a reflection of their common descent. . The skeletalcomponentsof mammalianforelimbs are a good example (see Campbell,Figure 22.9) = Although the limbs are usedfor different functions, it is obvious that the same skeletalelementsare present. = It is logical that whetherthe forelimb is a foreleg,wing, flipper, or arm, the basic similarity is the consequenceof descent from a common ancestorand that the limbs have been modified for different functions. They are homologous structures. Homologous structures: Structuresthat are similar becauseof common ancestry . Other evidence from comparative anatomy supportsthat evolution is a remodeling process in which ancestral structures that functioned in one capacity have become modified as they take on new functions. . Some homologous structuresare vestigial organs. Vestigial orgons : Rudimentary structuresof marginal or no use to an organism . Vestigial organsare remnants of structuresthat had important functions in ancestralforms but are no longer essential. ' Example: The remnants of pelvic and leg bones in snakesshow descent from a walking ancestor,but have no function in the snake. Modification:A DarwinianView of Life 337 Chapter22 DescentWith Becauseit would be wasteful to continue providing blood, nutrients, and spaceto structuresthat no longer have a major function, vestigial organs serve evidenceof evolution by natural selection' . 4. Comparative embrYologY closely related organismsgo through similar stagesin their embryonic development. . Vertebrate embryos (fishes, amphibians, reptiles, birds, mammals) go through an embryonic stage in which they possessgill slits on the sidesof their throats(see Campbell,Figure 22.10). . As developmentprogresses, the gill slits develop into divergent structures characteristicof each vertebrateclass. . In fish, the gill slits form gills; in humans, they form the eustachian tubes that connect the middle ear with the throat. Comparative embryology often establisheshomology among structures,such as gill pou"'h.r, that becomeso alteredin later developmentthat their common origin is not apparentby comparing their fully developed forms' "ontogeny In the late nineteenthcentury,embryologistsdevelopedthe view that recapitulatesPhYlogenY'" . This view held that the embryonic developmentof an individual organism (ontogeny) is a replay of the evolutionary history of the species . . @hylogenY). This is an extreme view; what does occur is a seriesof similar embryonic stagesthat exhibit the same characteristics,not a sequenceof adult-like stages. OntoBeny can provide cluesto phylogeny, but all stagesof development may becomemodified over the courseof evolution' 5. Molecular biologY An organism'shereditary backgroundis reflected in its genesand their protein products. . Siblingshave greater similarity in their DNA and proteins than do two unrelatedorganismsof the same species. . Likewise, two speciesconsideredto be closely related by other criteria shouldhave a greater proportion of their DNA and proteins in common than more distantlyrelatedspecies. Molecular taxonomistsuse a variety of modern techniquesto measurethe degree of similarity among DNA nucleotidesequencesof different species. . The closer two speciesare taxonomically, the higher the percentage of common DNA; this evidencesupportscommon descent' . Common descentis also supportedby the fact that closely related species also have proteins of similar amino acid sequence(resultingfrom inherited genes). of monomers . If two specieshave many genesand proteins with sequences a common from copied been have must sequences the that maich closely, ancestor. Molecular biology has also substantiatedDarwin's idea that all forms of life are relatedto someixtent through branchingdescentfrom the earliestorganisms(see Campbell,Figure22.1l)' 338 Unit IV Mechanisms of Evolution Even taxonomically distant organisms(bacteria and mammals) have some proteins in common. For example, cytochrome c (a respiratory protein) is found in all aerobic species.Cytochrome c moleculesof all speciesare very similar in structure and function, even though mutations have substitutedamino acids in some areas of the protein during the course of evolution. Additional evidence for the unity of life is the common genetic code. This mechanism has been passedthrough all branches of evolution since its beginning in an early form of life. B. What is theoretical about the Darwinian view of life? Dismissing Darwinism as 'Just a theory" is flawed because: . Darwin madetwo claims: l. Modern speciesevolved from ancestralforms. 2. The mechanism for evolution is natural selection. ' The conclusion that specieschangeor evolve is basedon historical fact. What, then, is theoretical about evolution? . Theories are conceptual frameworks with great explanatory power used to interpret facts. . That speciescan evolve is fact, but the mechanism Darwin proposed for that change-natural selection-is a theory. Darwin used this theory of natural selection to explain facts of evolution documentedby fossils, biogeography,and other historical evidence. In science,"theory" is very different from the colloquial use of the word, which comes closer to what scientistsmean by a hypothesis,or educatedguess. ' Unifuing concepts do not become scientific theories, unless their predictions stand up to thorough and continuoustesting by experiment and observation. . Good scientists, however, do not allow theories to become dogma; many evolutionary biologists now question whether natural selection alone can account for evolutionarv historv observedin the fossil record. REFBRENCES Campbell,N., et al. Biology.5th ed. Menlo Park, California:Benjamin/Cummings,1998. Futuyma, D.J. Evolutionary Biolog,,.2nd ed. Sunderland,Massachusetts:Sinauer,1986. Because it is broad in scope,this advancedtextbook is a good referencefor backgroundinformation. Mayr, E. The Growth of Biological Thought: Diversity, Evolution and Inheritance. Cambridge, Massachusetts:Harvard University Press, 1982. This is an excellent reference for historical perspective and could be a useful companion to the text for supplementing lecture material on many topics.